The "Go to classic results page." link at the bottom of the Modern search results page points to TENANT.sharepoint.com/search/results.aspx?k=xyz. It is missing /pages/. Should be TENANT.sharepoint.com/search/pages/results.aspx?k=xyz. I also tried on 2 different enterprise tenants. One of them has the same issue. The other points to /_layouts/15/osssearchresults.aspx?k=xyz and works fine.